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A CLASSIFICATION of the COMMUTATIVE BANACH PERFECT SEMI-FIELDS of CHARACTERISTIC 1: APPLICATIONS Eric Leichtnam View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Archive Ouverte en Sciences de l'Information et de la Communication A CLASSIFICATION OF THE COMMUTATIVE BANACH PERFECT SEMI-FIELDS OF CHARACTERISTIC 1: APPLICATIONS Eric Leichtnam To cite this version: Eric Leichtnam. A CLASSIFICATION OF THE COMMUTATIVE BANACH PERFECT SEMI- FIELDS OF CHARACTERISTIC 1: APPLICATIONS. Mathematische Annalen, Springer Verlag, 2017, 369 (1-2), pp.653-703. 10.1007/s00208-017-1527-1. hal-02164098 HAL Id: hal-02164098 https://hal.archives-ouvertes.fr/hal-02164098 Submitted on 24 Jun 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. A CLASSIFICATION OF THE COMMUTATIVE BANACH PERFECT SEMI-FIELDS OF CHARACTERISTIC 1: APPLICATIONS. ERIC LEICHTNAM A la mémoire de Tilby. Abstract. We define and study the concept of commutative Banach perfect semi-field (F, ⊕, +) of characteristic 1. The metric allowing to define the Banach structure comes from Connes [Co] and is constructed from a distinguished element E ∈ F satisfying a structural assumption. We define the spectrum SE(F) as the set of characters φ : (F, ⊕, +) → (R, max, +) satisfying φ(E)=1. This set is shown to be naturally a compact space. Then we construct an isometric isomorphism of Banach semi-fields of Gelfand-Naimark type: 0 Θ : (F, ⊕, +) → (C (SE(F), R), max, +) X 7→ ΘX : φ 7→ φ(X)=ΘX (φ) . In this way, F is naturally identified with the set of real valued continuous functions on SE(F). Our proof relies on a study of the congruences on F and on a new Gelfand-Mazur type Theorem. As a first application, we prove that the spectrum of the Connes-Consani Banach algebra of the Witt vectors of (F, E) coincides with SE(F). We give many other applications. Then we study the case of the commutative cancellative perfect semi-rings (R, ⊕, +) and also give structure theorems in the Banach case. Lastly, we use these results to propose the foundations of a new scheme theory in the characteristic 1 setting. We introduce a topology of Zariski type on SE(F) and the concept of valuation associated to a character φ ∈ SE(F). Then we come to the notions of v−local semi-ring and of scheme. Contents 1. Introduction. 1 2. Definitions, first properties and the spectrum SE(F). 5 2.1. Definition of a commutative Banach perfect semifield of characteristic 1. 5 2.2. Spectrum of a commutative perfect semi-field F of characteristic 1. 11 3. Congruences on (F, ⊕, +). 13 3.1. Algebraic properties of congruences and operations on quotient spaces. 13 3.2. Maximal Congruences. An analogue of Gelfand-Mazur’s Theorem. 18 4. The Classification Theorem ??. Applications. 20 5. Determination of the closed congruences of (F, ⊕, +). 25 6. About commutative perfect cancellative semirings of characteristic 1. 27 6.1. Cancellative semi-rings and congruences. 27 6.2. Embeddings of a large class of abstract cancellative semi-rings into semi-rings of continuous functions. 29 6.3. Banach semi-rings. An analogue of Gelfand-Mazur’s Theorem. 32 Date: June 24, 2019. 1 2 ERIC LEICHTNAM 7. Foundations for a new theory of schemes in characteristic 1. 34 7.1. A Topology of Zariski Type on SE(F). 35 7.2. Valuation and localization for semi-rings. 38 7.3. Locally semi-ringed spaces. Schemes. 41 References 43 1. Introduction. We adopt the following definition of semi-ring (R, ⊕, +) of characteristic 1. It is a set R endowed with two commutative and associative laws satisfying the following conditions. The law ⊕ is idempotent (e.g. X ⊕ X = X, ∀X ∈ R) whereas the law +, which plays the role of a multiplicative law, has a neutral element 0 and is distributive with respect to ⊕. The semi-ring R is said to be cancellative if X + Y = X + Z implies Y = Z. If for any n ∈ N∗, X 7→ nX is surjective from R onto itself, then (R, ⊕, +) is called a perfect semi-ring. If (R, +) is a group then (R, ⊕, +) is called a semi-field and we denote it by F rather than R. Morally, the first law ⊕ is the new addition and the second law + is the new multiplication. But, being idempotent, ⊕ is very far from being cancellative. The theory of semi-rings is well developed (see for instance [Go], [Pa-Rh]). It has impor- tant applications in various areas: tropical geometry ([Ak-Ba-Ga], [Br-It-Mi-Sh]), computer science, tropical algebra ([Iz-Kn-Ro], [Iz-Ro], [Le]), mathematical physics ([Li]), topologi- cal field theory ([Ba]). In the references of these papers the reader will find many other interesting works in these areas. Thanks to Connes-Consani, the semi-ring theory of characteristic 1 plays also a very interesting role in Number Theory: [Co-C0], [Co-C1], [Co-C2], [Co-C3]. For instance they constructed an Arithmetic Site (for Q) which encodes the Riemann Zeta Function. They analyzed the contribution of the archimedean place of Q through the semi-field (R, max, +). There, the natural multiplicative action of R+∗ on R is denoted × and is an analogue of the Frobenius, whereas the (internal) multiplicative operation of (R, max, +) is +. In constructing Arithmetic sites of Connes-Consani type for other numbers fields K, Sagnier( [Sa]) was naturally lead to more general semi-fields of characteristic 1. They are associated to some tropical geometry. For example, in the case K = Q[i], [Sa] introduced the set Rc of compact convex (polygons) of C which are invariant under multiplication by i. Then he analyzed the contribution of the archimedean place of Q[i] through the semi-field of fractions of the cancellative perfect semi-ring: (Rc, ⊕ = conv, +) . Furthermore, Connes-Consani ([Co-C0], [Co]) have developed a very interesting theory of Witt vectors in characteristic 1. Following [Co] one defines the analogue of a p−adic metric on a perfect semi-field F in the following rough way (see Section 2 for details). First recall the well-known partial order on F defined by: X ≤ Y if X ⊕ Y = Y . Next, it turns out that F carries a natural structure of Q−vector space. Assume then the existence of E ∈F such that for any X ∈ F there exists t ∈ Q+ such that −tE ≤ X ≤ tE (see Assumption 2.1). In the sequel we fix such an E, it is indeed a structure constant of in our framework. COMMUTATIVE BANACH PERFECT SEMI-FIELDS OF CHARACTERISTIC 1. 3 Denote by r(X) ∈ R+ the infimum of all such t ∈ Q+ and assume moreover that r(X)=0 implies X =0 (see Assumption 2.2). Then (X,Y ) 7→ r(X − Y ) defines a distance d on F, we say that (F, ⊕, +) is a Banach semi-field if F is complete with respect to d. The norm r(X) satisfies also the following inequality of non-archidemean type: ∀X,Y ∈F, r(X ⊕ Y ) ≤ max(r(X),r(Y )) . Thus there is also an analogy between the concept of commutative perfect Banach semi-field of characteristic 1 and the one of perfect nonarchimedean Banach field. The latter has been investigated by deep works in the nonarchimedean world ([Be], [Ke], [Ke-Li]), Therefore, it seems relevant to try first to classify the commutative Banach perfect semi- fields (F, ⊕, +) of characteristic 1. To this aim, it is natural to associate to (F, ⊕, +) the set SE(F) of the characters φ : (F, ⊕, +) → (R, max, +) such that φ(E)=1. More precisely, φ satisfies the following: ∀X,Y ∈F, φ(X + Y )= φ(X)+ φ(Y ), φ(X ⊕ Y ) = max(φ(X),φ(Y )) . We endow SE(F) with a natural topology T making it a compact space, we may call it the spectrum of F. Then we construct (see Theorem 4.1 for details) an isometric isomorphism of Banach semi-fields of Gelfand-Naimark type: 0 Θ:(F, ⊕, +) → (C (SE(F), R), max, +) (1.1) X 7→ ΘX : φ 7→ φ(X)=ΘX (φ) . Thus X ∈F is identified with the continuous function ΘX on the spectrum SE(F). Now we describe briefly the content of this paper. In Section 2.1 we define the concept of commutative perfect semi-field (F, ⊕, +) of char- acteristic 1 and introduce some basic material. Actually we need to introduce for F the concept of F −norm which is more general than the one given by r(X − Y ). By definition a F −norm kk satisfies: kX ⊕ Y − X′ ⊕ Y ′k ≤ max(kX − X′k, kY − Y ′k) . If (F, kk) is complete then, guided by [Co], we construct a continuous Frobenius action of (R+∗, ×) on F and we obtain a natural real vector space structure on F. We also prove various Lemmas showing that r(X) has morally the properties of a spectral radius. In Section 2.2 we introduce the set SE(F) of the normalized characters and endow it with the weakest topology (called T ) rendering continuous all the maps φ 7→ φ(X), X ∈ F. In Theorem 3.3 we prove that (SE(F), T ) is a compact topological space. In Section 3 we define and study algebraically the notion of congruence for (F, ⊕, +). A congruence ∼ is the analogue in characteristic 1 of the concept of ideal in classical Ring theory.
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